When comparing 5-inch and 6-inch gutters, it’s important to remember that there isn’t a universally ‘correct’ answer. Each building is unique and every project has its own specifications. There are big differences, however. In the popular K-style, for example, 6-inch gutters can hold almost 50% more water than 5-inch gutters. Very generally speaking, 6-inch gutters are used for commercial buildings, whereas residences use 5-inch gutters. There are many factors to consider, though.
Roof size is consideration. Simply put, in a storm, more water will come off of a larger roof. As the gutter is meant to carry water away from the building, more water will require a larger gutter. The slope of the roof can also impact how much water comes down at once. Even a smaller roof may benefit from 6-inch gutters if the slope is steep enough. The climate of the area should be considered, as larger gutters may obviously be more appropriate for areas that get a lot of rainstorms. The fascia board may also be a factor, as some fascia boards aren’t able to support larger gutters. Conversely, if you have large fascia boards, you may want larger gutters for aesthetic reasons, to keep everything looking in proportion.
As you can see, there really is no universal rule regarding gutter size, and there are many individual factors to consider.